This invention relates to vent valve devices and particularly to a means for temporarily disabling a vent valve device when conducting a single car test in accordance with the new single car test procedure specified by the A.A.R. (Association of American Railroads) for checking the general condition of the brake system of "in date" railway freight cars and cars having undergone "periodic repairs". A single car test device enables this test procedure to be accomplished without removal of any of the air brake components from the car.
In one phase of this test procedure, referred to as the service stability test, the car control valve is checked to assure that it does not go to emergency during a service rate of reduction of brake pipe pressure. Typically, this test was performed satisfactorily without having to isolate the car vent valve device, when equipped with such. Recently, a revised single car test procedure was instituted, however, for carrying out the service stability test for control valves on long cars having over 75 feet of brake pipe. Essentially this revision of the test procedure requires a faster rate of brake pipe pressure reduction, which causes the car vent valve device to actuate. In order to prevent the vent valve from influencing the rate of brake pipe pressure reduction, when inadvertently actuated during the new single car test, the vent protector was removed and reinstalled in the vent valve exhaust port in its closed position to block the exhaust of brake pipe pressure. In the industry standard #8 and KM-2 vent valves, this proved to be an effective means of preventing an actuated vent valve from dumping brake pipe pressure and thereby adversely affecting the service stability test.
In attempting to employ this same procedure with the recently developed VX Vent Valve disclosed in U.S. Pat. No. 4,974,911, it was found that even with the vent valve exhaust port plugged, a sudden 1 psi drop in brake pipe pressure still occurs, due to the particular design of the exhaust valve diaphragm causing relatively high volumetric displacement when the vent valve is actuated. While this brake pipe pressure drop is beneficial in terms of transmitting an emergency application in actual operation, it adversely affects the control valve stability during the new service stability test.